1. Field
The invention herein is related to telecommunication systems, particularly to wireless communications devices, and most particularly to mobile communication stations.
2. Description of the Relevant Art
Within 20 years, over a billion wireless service subscribers made mobile voice and mobile data services integral parts of their business and social life. In many parts of the world, wireless communications are more prevalent—and more reliable—than fixed or land-based communications services, with international roaming services being available in over 160 countries. Thus, mobile wireless communications quickly are becoming essential tools in modern society.
By way of background, a communication is the transfer of messages, or information-bearing signals representing signs, writing, images and sounds, or intelligence of any nature, according to agreed conventions. Herein, transferring a message may include sending a message, receiving a message, or both; with the logical meaning assigned to one or more messages being called content. Communication is effected by least two communication endpoints. A host is communication endpoint attached to a communication network that can be a server (producer) or a client (consumer) of messages. A host generally executes application programs on behalf of one or more user, integrated, or embedded systems, or communication content provider, employing communication services in support of this function. These executed programs are among the processes, or active elements, in a host. A intermediate node also can be a communication endpoint, for example, when the node produces or consumes messages, and not merely relays or re-transmits the messages to another host or node. In general, a communication link represents the association between contiguous connecting points, or between an endpoint and a contiguous connecting point. Two or more hosts communicate over a concatenation of links, called a connection, which represents the physical and logical association between endpoints, and which provides the capability of transferring information between endpoints. A connection may incorporate links using a variety of communication media.
Telecommunication is the transfer of messages by wire, radio, optical, or other electromagnetic systems. Telecommunication service providers, which may be public or private entities, make available telecommunication services to users by subscription, and enable users to transfer messages. Users may be humans or machines, which are designated by humans to use the telecommunication services. Wireless telecommunication services include those in which at least one communication link between provider and user employs radio, optical, or other electromagnetic systems. Such services may be fixed or mobile, and may transfer messages using both terrestrial and satellite links. A wireless link generally conveys messages according to wireless communication protocol, i.e., a formal set of conventions governing the format and timing of message exchange between two communication endpoints. A protocol typically is implemented using a carrier access method, which also can be represented by a formal set of conventions governing the physical signal characteristics, and method of signal transmission over a physical air interface, i.e., the Um interface.
By implementing a set of network capabilities, which are defined by standardized protocols and functions, a service provider can configure a network to participate in an integrated services network offering diverse, distributed communication services. When a demand for a telecommunication service is invoked by a user, then the particular instance of the service also may be called a communication. Such communications may enlist the services of multiple, internetworked service providers, and may transfer messages over many links. Mobile wireless communication services (mobile services) are adapted for applications in which at least one user is capable of changing location during a communication (i.e., a mobile user). Furthermore, mobile services can include telecommunication provider services that may be delivered using fixed, wireless, and mobile communications services, or a combination thereof. The concept of mobile communications represents: (1) the ability of a user to access telecommunication services at any terminal from different locations; (2) the capability of the network to provide those services according to the user's service profile; (3) the network capability to identify and locate the terminal associated with the user for the purposes of addressing, routing and charging of the user's calls; and (4) the ability to do so while in motion.
Mobile telecommunication service users gain access to these services by executing a service level agreement with the mobile service provider. A significant aspect of this agreement may include the Quality of Service (QoS) provisions relating to the delivery of services and content, with selected network traffic receiving better service than others, typically at a higher cost. QoS parameters may include a broad range of attributes specified by content service providers, network service providers, and users, including cost; reliability; priority; protection from unauthorized access; transmission and delivery error rates; system throughput; expected delays and variations; basic service availability; and availability of alternative services and carrier access methods. Some QoS parameters can be set in advance and some may be dynamically adapted to, or selected to accommodate, existing conditions.
The improved service may be effected by providing dedicated bandwidth, controlled jitter and latency, and improved loss characteristics, but usually not at the expense of disrupting other flows having lower priority. QoS can refer to the level of service to which provider and subscriber agree, as well as to the cost that a subscriber is willing to pay to receive enhanced services. For example, a mobile user may subscribe to standard level services providing text and voice for a particular recurring fee, but may agree to pay an additional per-use charge for premium services, such as multimedia and international roaming. Each service at each level may require a different QoS level.
For many applications, particularly those related to the transmission of text or files, in which the associated messages arrive on a “best efforts” basis and often of order, such an impairment may not be perceived by the recipient or, if it is, the QoS reduction may not be significant or even noticeable. However, in isochronous (delay-sensitive) applications, such as voice, video, and multimedia, the delays in a mobile environment may have a greater perceptual impact upon the user and, thus, the satisfaction of the user with the service. For example, voice-over IP service (VoIP) has very tight delay constraints, i.e., a small delay budget. In general, voice quality will start to degrade if the round trip delay exceeds 250 ms. Although an overall delay budget may be brought to bear upon the end-to-end path, much of the time budget may be consumed by the complexities that the traffic encounters to make the last “hop” to the end user device.
The basic instrument that is used to access mobile wireless services is the mobile station (MS). A base station (BS), or base transceiver station, is a fixed station that employs a radio transceiver to communicate with MSs. Traffic is the set of messages communicated between an MS and a BS, which is transported over the communication network. The communication path between an MS and a BS, used transport user messages and control signals is called the traffic channel, or channel. The traffic channel may include an uplink, or a forward traffic path, between the MS and the BS, and a downlink, or reverse traffic path, between the BS and MS. One or more physical channels can cooperate to act as a single logical channel and vice versa, depending upon the services to which the user has subscribed. Each traffic channel has a set of transmission formats, i.e., a radio configuration, that is characterized by physical layer parameters such as transmission rates, modulation characteristics and spreading rate. The channel forms a communication link over the air interface, also called the Um interface, between an MS and a BS.
Typically, one or more BS are connected with a Mobile Switching Center (MSC), an automatic system which constitutes the interface for user traffic between the cellular network and other public switched networks, or other MSCs in the same or other cellular networks. In general, an MSC and its associated BSs can be considered as a single functional entity, the BS/MSC. The BS/MSC cooperate to form one or more clusters of cells that provide wireless services, for example, over a metropolitan region or transportation corridor. Multiple BS/MSC can be coupled to form a service network or system, covering a substantial geographic area.
Frequently, the BS in a particular service network employ a common wireless configuration or mode of operation, hereinafter called a carrier access method. There are many carrier access methods currently in use. On the user/subscriber side, an MS often is configured to perform in one carrier access method, although MS accommodating multimode communications are becoming more common, as subscribers demand increasing flexibility with wireless services while roaming away from their home wireless network. A multimode MS is one, which can communicate using two or more carrier access methods.
On the “land” side of the Um interface, mobile service providers connect their wireless networks to other wireless networks, including those using different carrier access methods, and to globally-available integrated services networks. Because two communicating users may be interconnected by heterogeneous media, a single communication can be transmitted over wire, fiberoptic, and wireless media using multiple, medium-related communication protocols. In general, hosts are heterogeneous as well, with some hosts communicating one or more types of digital information, including data, text, voice, facsimile, video, multimedia, and a combination thereof.
In principle, mobile networking can allow the roaming user to receive mobile services in much the same way as at home. When a user powers on an MS, the MS provides the local wireless system with mobile or subscriber identification information; the local system uses the wireless network to find and inform the user's home system of the user's current serving MSC. This allows calls to the mobile that arrive at the home system to be redirected to the current serving system. From the MS user's point of view, a home MSC may appear as a local calling area, although local-calling areas, as designated by the service provider, may be larger than a single MSC.
Increasingly, wireless and mobile devices have at least a portion of their connection linked over public networks, including the Internet. The Internet is a loosely-organized international collaboration of autonomous, interconnected, packet-based networks, including wireless networks, which may generally be represented by the multilayer architecture, service descriptions, and protocols described by ITU-T Rec. X.200: “Information Technology—Open Systems Interconnection—Basic Reference Model: The Basic Model,” International Telecommunication Union, July 1994 (OSI Basic Reference Model).
These networks support host-to-host communication through voluntary adherence to open protocols and procedures defined by standards and practices, which are grounded in a few underlying principles. These principles tend to be inherent in Internet-related protocols. In general, these protocols give “end-to-end” responsibility for the integrity, message flow management, and the security, of communication to the end hosts, thereby making the communication substantially “transparent” to intermediate systems. Internet protocols are well-known in the networking and telecommunications arts, and are identified in IETF Standard STD0001: “Internet Official Protocol Standards,” J. Reynolds, R. Braden, S. Ginoza, A. De La Cruz, Eds., November 2002 (also called IETF RFC3300). The networks constituent of, and coupled to, the Internet generally are interconnected using packet-switching computers called “routers,” “gateways,” or “intermediate systems.” To improve robustness of the communication system, gateways often are designed to be stateless, forwarding each IP message independently of other messages. Consequently, redundant paths can be exploited to provide robust service in spite of failures of intervening gateways and networks. To communicate using the Internet system, a host typically implements at least one protocol from each layer of the layered Internet protocol suite. The Application Layer is the topmost layer of the Internet protocol suite. Two categories of application layer protocols include user protocols that provide service directly to users, and support protocols that provide common system functions. Application layer protocols employ TCP to provide their transport layer communication services.
The predominant group of protocols used on the Internet is the collection of protocols called the Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite. The policy and implementation details of both the IP and the TCP protocols are well-known and are described respectively in, for example, IETF Standard STD0007: “Transmission Control Protocol,” J. Postel, September 1981 (also called IETF RFC0793) and IETF Standard STD0005: “Internet Protocol,” J. Postel, September 1981 (includes IETF RFC0791, IETF RFC0792, IETF RFC0919, IETF RFC0922, IETF RFC0950, and IETF RFC1112). STD0003 also describes link, IP, and transport layer communication protocols, as well as application and support protocols. In general, the TCP/IP protocol suite conveys information in datagrams, or structured blocks of data units. As used herein, however, the term message will apply to datagrams, packets, and frames, as well as any other unit or structured block of communication. TCP/IP-based traffic allows messages to be conveyed through the Internet and a mobile network, using packet switching with a substantial degree of transparency. To achieve the desired connectivity between source and destination, TCP is used at the Transport Layer (Layer 4) in combination with the Internet Protocol at the Network Layer (Layer 3). Typically, successive messages are transferred over this routing path.
TCP is intended to be a reliable connection-oriented transport layer service that provides end-to-end reliability, resequencing, and flow control. With TCP, a logical connection between source and destination is mapped prior to data transmission, a process called routing. IP is a connectionless protocol, in that no logical connection between an endpoint and a network exists prior to data transmission. The IP protocol provides no end-to-end delivery guarantees for messages. Each message is identified by a destination address, namely the IP address, and can travel to the destination substantially independently of other messages in the transmission. Messages contend for the use of network resources as they are routed through the network and, thus, may arrive at the destination host damaged, duplicated, out of order, or not at all.
The layers above IP, such as TCP, or an application layer support protocol, are responsible for reliable delivery service when it is required. TCP uses IP to carry its data end-to-end, and then receives, reorders, repairs, and requests re-transmission of the messages conveyed by IP, as necessary. Because of the transparency principle, messages can flow essentially unaltered throughout the network using TCP/IP-based protocols, and their source and destination IP addresses could be used as unique labels for the hosts. Moreover, end-to-end connectivity tends to reduce the impact of single-point failures in a routing path and to simplify securing message transport.
However, despite its simplicity, advantages, and ubiquity, the TCP/IP protocol suite evolved using basic assumptions that can be problematic in a mobile networking environment. For example, the IP suite was designed with the assumption that devices attached to the network are stationary. In addition, higher layer protocols, such as TCP, inherit this assumption, so that network connection properties are shared among many entities, and across network protocols, transport protocols, and applications. For example, TCP uses IP addresses to identify its connection endpoints. However, applications use sockets for their network I/O, with a socket typically being composed of an IP address and a specific TCP port number. Thus, during a communication, a mobile station IP address may change, which can break the associated TCP connections and can result in lost messages and an undesirable disruption of service.
To manage message flow over a connection, the TCP protocol effects congestion control and congestion avoidance employing both reactive and preventive techniques, such as those described by IETF Standard STD0003: “Requirements for Internet Hosts,” R. Braden, Ed., October 1989 (also called IETF RFC1122 and ETF RFC1123). With these techniques, a TCP sender can adapt its use of network connection capacity, based on feedback from the TCP receiver. Absent an explicit congestion notification, though, a sending host may use a congestion window technique to control the flow of messages to the recipient. In general, a congestion window can be representative of the number of messages that can be correctly transmitted to a receiver, over a given period.
With one popular TCP congestion control strategy, the sender halves the congestion window size with each window transmitted with an error, i.e., at least one message in the window is deemed lost or damaged. For each subsequent message window that transmitted error-free, the sender can re-open the congestion window by one additional message. However, if additional errors are encountered before the congestion window recovers completely from a 50% reduction in size, even more 50% reductions in window size may be triggered, and the effect can be a “downward spiral,” until the congestion window collapses, (i.e, transmission halts). For batch-mode communications, in which the messages can be re-transmitted without significant consequence, a strategy of this type can be beneficial to the network as a whole and not necessarily detrimental to either sender or receiver. In a mobile environment, this strategy can lead to an undesirable or even unacceptable results, because delayed packets in delay-sensitive applications may be discarded.
Latency is an undesirable physical phenomenon, which may arise from many sources; the effect of these sources often is additive. For example, overall network delay can be a function of the capacity of the links in the network, the overall amount of queuing and switching, and the format processing that may occur as the packets transit the network. Latency will differ with the carrier access method used, relevant environmental factors, and the spatial-geographic path of the link. Also, the more frequently a message is buffered while in transit, the more quickly the latency budget is expended. Often, between 50 ms to 150 ms may be consumed merely to cross the network connection, excluding latency, which may be introduced by the wireless link and the endpoint devices themselves. Because the message flow impairment for some applications may be less acceptable than for others, the QoS perceived by a user may suffer unless mechanisms are provided to grant and enforce a higher QoS for delay-sensitive messages, or to avoid or minimize delay.
In another example, end-to-end latency (or end-to-end delay) is the sum of the delays at the different network devices and across the network links through which traffic passes, from the sending endpoint to the receiving endpoint. In telephony terms, latency is the measure of time it takes the talker's voice to reach the listener's ear. Each additional delay may reduce the remaining time budgeted for message transmission. As the end-to-end delay of a conversational voice message surpasses 50 ms, echo becomes increasingly noticeable. As the delay surpasses 200 ms, speaker and listener can become un-synchronized, with either talking or pausing at once. Beyond 400-700 ms, speech may become unintelligible. A generally accepted limit for one-way, end-to-end delay for real-time voice applications is about 250 ms, with a 150 ms budget cap being desirable.
Current MS tend to serve as host for attached terminal devices. These MS may employ queueing, address translation, message security functions, and the like, serving “middlebox” functions on behalf of their client terminal device, instead of primarily acting as a message router. Thus, endpoint host devices, including MSs, can introduce delays, for example, by performing significant buffering and message transcoding, and by introducing algorithmic delays from signal compression (sender) or decompression (receiver). Receiving devices may attempt to offset delay variability (jitter) by buffering messages, introducing even more delay. Unfortunately, sensitive voice and video applications may involve a substantial amount of signal processing time, further adding to overall host-imposed latency. However, although latency is undesirable, some contributing factors may be more amenable to control than others.
Typically, mobile applications operate in a highly variable context, rarely encountered in the wired portions of a connection. For example, mobile systems can encounter air interface link problems, which increase the risk of delayed or lost packets. The link problems include variable bandwidth and signal strength, bursty noise, handoff losses, interference, and fading due to time dispersion from multipath transmissions, and to Doppler signal spreading. Each link problem contributes to an increase in the rates of bit errors and packet losses, often increasing latency and degrading service indirectly, through the initiation of TCP congestion protocols, or directly, for example, through channel failure.
Consequently, it is undesirable for a host to perform proxy-like services for client devices, which require additional buffering and processing. Exemplary host proxy services, such as message queueing, address translation, and message forwarding services, can introduce enough latency to make the perceived QoS provided by the MS unsatisfactory to the user.
In addition, TCP congestion control evolved under the assumption that network congestion is the only cause for message loss. In a mobile environment, TCP message delays and losses can be misinterpreted by the TCP congestion algorithms, leading to an unnecessary reduction in link bandwidth utilization and increase in message latency. In current IP networks, all messages may be treated alike. Under peak loads and congestion, voice frames can be dropped equally with data frames. The data frames, however, are not time sensitive, and dropped data packets can be appropriately corrected by retransmission. Lost voice packets, however, cannot be dealt with in this manner. The impact of loss varies with the application, but for interactive voice, or high-quality streaming video applications, it is desirable to keep the fraction of lost packets to less than about 5%.
Complicating matters, roaming users may carry with them an collection of electronic devices to meet their needs, including, for example, voice handset, pager, personal digital assistant, video terminal, laptop computer, intelligent print output, and associated accessories. With few exceptions, these devices are capable of communicating using standard physical interface technology. This interface technology includes, for example, ITU-T X- and V-series interfaces; Peripheral Components Interconnect (PCI/PCI-X) interfaces; Universal Serial Bus (USB) connection interfaces; and the many open and proprietary interface standards promulgated, for example, by professional organizations (IEEE, IEC, ANSI, etc.) and by private industry (e.g., BLUETOOTH®). Current MSs offer limited connectivity to these standard interfaces on the Rm side, sometimes leaving the user with a Hobson's dilemma when choosing from their personal collection of electronic devices. When these mobile terminals are connected to an MS, the MS typically acts as the destination host device, performing proxy-type “middlebox” services including address translation and message routing for each connected mobile terminal ingress messages. Depending upon the mobile terminal, the MS also may transcode and convert the data into the desired mobile terminal device format. As noted above, this additional processing can be deleterious to the overall end-to-end time budget, potentially resulting in a perceptibly degraded Quality-of-Service.
The diverse mobile communication environment of terminal devices, device interface link standards, carrier access methods, telecommunication services, and Quality of Service requirements, may place great and varied demands on an MS. Thus, it is desirable to provide an MS that is capable simultaneously supporting multiple mobile terminals over multiple wireless network technologies, with a reduced impact on Quality of Service requirements.